1. Field of the Invention
The invention provides a process for electrical energy production with the aid of a fuel cell.
2. Description of the Related Art
Fuel cells as energy converters have been studied very intensively in the recent past both for mobile and for stationary applications. Fuel cells enable the electrochemical conversion of fuel gases and oxygen into oxidized products and electrical energy. The difference from traditional chemical processes consists of performing reduction and oxidation of the components, separately, at two electrodes. Chemical reaction of the reactants at the electrodes occurs because ionic conduction is ensured via a gas-tight electrolyte, and the transport of electrons takes place only via an external circuit.
Hitherto, substantially hydrogen and, with a much lower efficiency, methanol, have been studied as fuels. Oxygen and air have been considered as oxidizing agents. The less favorable efficiency of the direct methanol fuel cell is due to the intermediates of methanol oxidation being very strongly adsorbed at the anode. A similar effect can be observed in the case of a hydrogen fuel cell when the fuel gas is contaminated with carbon monoxide. The drop in efficiency due to the presence of carbon monoxide is then a function of the carbon monoxide concentration and the operating temperature.
The purity of the hydrogen, and in particular the presence of carbon monoxide, thus has a large effect on the electrical efficiency. Ultra-pure hydrogen can be obtained directly from water and electrical energy. This production process, however, is sensible only in certain cases, for example solar energy, and is therefore used in that context. For industrial applications of fuel cells, the hydrogen is obtained from fossil fuels. Typical representatives of these fuels are natural gas, methanol and aliphatic or aromatic hydrocarbons, as well as mixtures thereof, such as, for example, petrol and diesel oil. In principle, it is also possible to produce the hydrogen-containing fuel gas biologically and directly as synthesis gas and to work it up in an appropriate manner for use in a fuel cell. Methanol can also be produced biologically, for example, with the aid of methylotrophic yeasts.
These energy carriers can be converted, for example by steam reforming, into a gas mixture consisting of residual fuel, carbon dioxide, carbon monoxide, and hydrogen. Downstream of the reformer, the gas contains about 5 vol. % carbon monoxide, in the case of reforming methane. This concentration is unsuitable both for the currently used low-temperature fuel cells based on a polymer electrolyte membrane (PEMFC=Polymer Electrolyte Membrane Fuel Cell), and for the phosphoric acid fuel cell (PAFC=Phosphoric Acid Fuel Cell).
The first purification route is associated with a high energy demand, due to PSA, and also requires extensive equipment. CO hydrogenation then leads to a gas with very low residual concentrations of carbon monoxide. As a result of the hydrogenation reaction, however, the amount of hydrogen produced is decreased.
For a PAFC which operates as a stationary system at about 200xc2x0 C., a concentration of up to about 1 vol. % carbon monoxide can be tolerated in the fuel gas. In order to maintain this value, carbon monoxide contained in the reformate can be reacted with water to give carbon dioxide and hydrogen, in the presence of suitable catalysts, in accordance with the following chemical equation:
CO+H2O⇄H2+CO2 xcex94H greater than 0xe2x80x83xe2x80x83(1) 
The reaction in accordance with chemical equation (1) is called carbon monoxide conversion, or CO conversion, in the following. In English, the expression xe2x80x9cwater gas shift reactionxe2x80x9d is often used to describe this process.
Carbon monoxide conversion is normally performed in a two-step process. In the first process step, so-called high temperature CO conversion (high temperature water gas shift, HTS) is performed at temperatures between 360 and 450xc2x0 C. on Fe/Cr catalysts. In the subsequent, second step, low temperature CO conversion (low temperature water gas shift, LTS) is performed at temperatures between 200 and 270xc2x0 C. on Cu/ZnO catalysts. Following the low temperature process step, in accordance with the thermal equilibrium, concentrations of less than 1 vol. % carbon monoxide are present in the fuel gas.
The membrane fuel cell (PEMFC), a system with an operating temperature of about 80xc2x0 C., is much more demanding with regard to CO concentration. Here, only values in the ppm range can be tolerated because carbon monoxide enters into very strong adsorptive interactions with the platinum particles on the electrode surface. Gas purification takes place, for example, in stationary units via pressure swing adsorption (PSA) of the carbon dioxide, and subsequent hydrogenation of the carbon monoxide to give methane, or by selective oxidation of the carbon monoxide to give carbon dioxide.
The first purification route is associated with a high energy demand, due to PSA, and also requires extensive equipment outlay. CO hydrogenation then leads to a gas with very low residual concentrations of carbon monoxide. As a result of the hydrogenation reaction, however, the amount of hydrogen produced is decreased.
Direct selective oxidation of carbon monoxide is currently the route which is followed, particularly in mobile applications. Here, a small amount of air is deliberately metered into the fuel gas stream. The atmospheric oxygen reacts with the carbon monoxide to give carbon dioxide. In this process arrangement, however, some of the useful gas hydrogen is also oxidized, so here again a drop in efficiency is observed.
Another process for removing carbon monoxide is the membrane technique with, for example, Pd/Ag membranes. The currently used membranes, however, are relatively expensive (limited palladium deposits), and require a high energy input in order to produce the pressure drop required. In addition, there are still some unsolved mechanical problems, and these can lead to low long-term stability.
Carbon monoxide conversion in accordance with chemical equation (1) is also used by certain microorganisms for the production of energy. See, for example, V. A. Svetlichny et al., xe2x80x9cCarboxidothermus hydrogenoformans gen. nov., sp. nov., a CO-utilizing thermophilic anaerobic bacterium from hydrothermal environments of Kunashir Islandxe2x80x9d, System. Appl. Microbiol. 14, 254-260 (1991), and M. Gerhardt et al., xe2x80x9cBacterial CO utilization with H2 production by the strictly anaerobic lithoautotrophic thermophilic bacterium Carboxydothermus hydrogenus DSM 6008 isolated from hot swampxe2x80x9d, FEMS Microbiology Letters 83 (1991) 267-272. According to DD 297 449 A5, this organism can be used to remove carbon monoxide from synthesis gas. DD 297 450 A5 describes a process for the microbial production of hydrogen and/or methane using this microorganism. Carboxidothermus hydrogenoformans is deposited at the German Collection of Microorganisms and Cell Cultures GmbH under number DSM 6008. The preceding references are incorporated by reference herein in their entirety.
The use of extremely thermophilic bacteria recently isolated from hot springs is particularly suitable for the process according to the invention. The strictly anaerobic bacteria from the strain Carboxidothermus hydrogenoformans DSM 6008 have been discovered only recently. They are capable of growing in aqueous media under an atmosphere of pure carbon monoxide. They form, in accordance with equation (1) and in addition to their cell substance, hydrogen and carbon dioxide as the only metabolic products, in equimolar amounts. The optimum conditions for the growth of these microorganisms are present at a temperature between 35 and 90xc2x0 C., at a pH between 5.0 and 8.0, and at a pressure from 1 to 10 bar. The enzyme CO-dehydrogenase is responsible for catalyzing the carbon monoxide conversion by Carboxidothermus hydrogenoformans.
The object of the present invention is to provide a process for electrical energy production with the aid of a fuel cell, in which the hydrogen required is obtained by reforming an organic fuel, and in which the carbon monoxide present in the reformate is converted to carbon dioxide and hydrogen by an energetically efficient process.
The object of the invention is achieved by a process for electrical energy production with the aid of a fuel cell, by reforming an organic fuel, removing carbon monoxide from the reformate, and reacting the hydrogen present in the reformate with oxygen, in a fuel cell, to thereby produce electrical energy. To remove the carbon monoxide from the reformate, the reformate is treated in a bioreactor which contains an enzyme or a thermophilic microorganism strain which metabolizes carbon monoxide under anaerobic conditions to give carbon dioxide and hydrogen.